The electrically erasable programmable read only memory (EEPROM) market has divided for historical reasons into four fairly distinct product segments. These include the EAROM, EEPROM, EEPROM-EAROMs and non-volatile SRAMs. Different types of devices have been developed for specific applications requirements in each of these segments. The low density (below 8 k) EAROMs have been used in such applications as consumer radio tuners, automotive engine controllers, etc. Medium density EEPROMs have been required by microprocessor based applications such as distributed systems or changeable program store. These parts have been developed with a focus on the high endurance and high speed requirements.
The four basic technologies used to manufacture electrically reprogrammable ROMs all utilize to some extent Fowler-Nordheim tunneling which is cold electron tunneling through the energy barrier at a silicon-silicon dioxide interface and into the oxide conduction band. The earliest electrically reprogrammable ROM process in the early 1970s utilized a metal-nitride-oxide silicon combination (MNOS) for the gate region of a P-channel storage cell producing devices called EAROMs (electrically alterable ROMs). The thin silicon dioxide layer allows charges to tunnel through when a voltage is applied to the gate. These charges are trapped in the silicon dioxide to silicon nitride interface and remain trapped there since the materials are high quality insulators. A double polysilicon process is used in a cell consisting of a MNOS transistor and a select transistor.
For programming, a negative voltage is applied to the source and drain while the substrate and gate are grounded. The potential at the central portion of the channel became almost the same as that of the drain and source so that tunneling electrons move from the silicon to the nitride through the thin oxide layer and the electrons are trapped in the nitride. This made the threshold of the MNOS transistor shift in the positive direction so that it conducted more difficult. In the mode of erasing, electrons are emitted from the traps in the nitride by applying a negative voltage to the gate electrode while the source and the drain are grounded.